1. Field of the Invention
This invention relates to a processor such as exposure device provided with a laser light source, repair device for wafer and mask using a laser beam or the like, and is particularly concerned with an exposure processor for printing a pattern formed on a mask onto a photosensitive substrate such as semiconductor wafer or the like through a projection optical system in a lithography process for production of semiconductor integrated circuits.
2. Related Background Art
A reduced projection exposure device in step-and-repeat system, or a so-called stepper functions centrally in a lithography process for the production of semiconductor integrated circuits. The stepper exposes a circuit pattern formed on a mask or reticle (hereinafter called reticle) onto a photosensitive substrate such as semiconductor wafer with a resist applied thereto or the like (hereinafter called wafer) sequentially at local domains through a reduced projection lens (hereinafter simply called projection lens). In recent years an integration of semiconductor integrated circuits has been enhanced more and more, and thus a requirement is such that the circuits will be formed in submicrons for minimum line width. Consequently, as is preferable for the lithography with a line width of submicrons, drawing the attention presently is a stepper using a laser emitted in the ultraviolet zone at high luminance and high output like KrF laser as a light source for exposure.
Meanwhile, a material usable on a projection lens with such ultraviolet zone laser whose wavelength is shorter than 365 nm (i line of Hg lamp) as an illumination light for exposure is limited to synthetic quartz, fluorite and the like. However, since a spectrum width of the laser is 0.3 to 0.4 nm or so in half-amplitude level, a chromatic abberation may result on the projection lens constructed, for example, only of a quartz lens. Consequently, an achromatic lens with glass materials combined in two kinds or more therefor will be used, however, there is a problem unavoidable in shape (size) and grinding characteristic of the crystal material such as fluorite or the like, and thus a design and manufacture of the lens are subjected considerably to limitations. Then, the projection lens may be fabricated of a single material of quartz lens instead of the aforementioned achromatic lens from using a light extremely narrow in a half-amplitude level of the spectrum width (or a light, for example, narrowed to a band 0.01 to 0.005 nm or below) as exposure light, and a difficulty in designing and producing the projection lens will be removed to a great extent.
Now, therefore, in the stepper provided with such laser, it is generally considered that an exposure of high resolution be realized by narrowing the laser wavelength to a band 0.003 to 0.005 nm through wavelength selectors such as Fabry-Perot etalon (hereinafter simply called etalon), grating and the like which are provided on the laser, and thus preventing a chromatic aberration from arising. However, the wavelength of a light oscillated from the laser fluctuates considerably large immediately after the laser starts oscillating. Further, the wavelength may fluctuate even after stabilized to a certain degree, as the wavelength selectors are subjected to an influence of mechanical vibration, temperature, atmospheric pressure and others. Accordingly, if such fluctuation of the wavelength, namely a variation of the center wavelength of a laser light to a set wavelength exceeds a predetermined tolerance (.+-.0.00l nm, for example), then since the projection lens for use on the laser has the abberation corrected only by a specific wavelength, fluctuations may arise on projection magnification, focal position, distortion and others according to the wavelength fluctuation, thus missing an exposure to be realized at high resolution. Consequently, it is important that a wavelength of the laser will be stabilized, or the center wavelength is matched with a set wavelength, and the wavelength variation is kept coming within a predetermined tolerance. Now, therefore, it is necessary that the wavelength selectors be controlled so as to stabilize the laser wavelength, thereby preventing fluctuations from arising on projection magnification, focal position, distortion and others due to the wavelength fluctuation. In case, for example, Fabry-Perot etalon is used as the wavelength selector, the etalon for wavelength selection will be inclined properly to an optical axis of the laser light through feedback control according to a variation of the laser light center wavelength to a wavelength set on a reference wavelength of mercury lamp or the like. A wavelength shift of the laser light is thus stabilized, and a reticle circuit pattern will be formed on a wafer at the best resolution characteristic at all times.
As mentioned above, in the stepper provided with this kind of laser, a stabilization of the laser light wavelength characteristic (or a stabilization of the center wavelength and wavelength width) is of importance, and a slight unstableness of the wavelength characteristic may cause fluctuations of projection magnification, focal position, distortion and others. For example, a laser light wavelength fluctuates heavily immediately after the laser recommences oscillation after shutdown for a predetermined time or longer, and hence a high precision (high resolution) of circuit pattern is not obtainable even from transferring the reticle circuit pattern onto a wafer in such state. Particularly, when the wavelength selectors are subjected to an influence of mechanical vibration and others during exposure to a wavelength fluctuation, a resist is sensitized unnecessarily as fluctuations arising on projection magnification, focal position and others due to the wavelength fluctuation, or a defective circuit pattern is printed on the wafer.
Consequently, when the stepper does not send an oscillation command (trigger signal) to a laser light source side for a predetermined period of time or over, namely, during replacement of a reticle or wafer, execution of alignment between the reticle and the wafer, the laser light source closes a shutter provided on a laser light outlet according to a command from the stepper, oscillates at a low frequency (1 to 2 Hz, for example) to a wavelength stabilization control, thereby preventing a wavelength fluctuation from arising at the time of restart of an oscillation command from the stepper (at the time of restart of exposure). An operation of the laser light source for oscillation regardless of a trigger signal from the stepper is defined as self-oscillation hereinafter.
In such system, the laser light source is operated for self-oscillation all the time while the laser light source is not used on the stepper side, therefore an arrangement is such that the self-oscillation is carried out at a constant low frequency so as not to shorten uselessly a lifetime of optical parts within the laser light source, parts for high tension control circuit (such as thyratron and others), or a lifetime of a gas for the laser.
However, there may be a case where a beam quality such as absolute wavelength, spectrum width, pulse energy or the like deteriorates unexpectedly (wavelength unlocked and so forth) for some reason such as, for example, a mechanical vibration of the laser light source or the stepper body, an incoming noise, a defective control of the etalon and others. Since the beam quality is monitored on the laser light side by spectroscope, detector and so forth, whether or not it has been deteriorated can be detected immediately by one pulse light emission, however, if the deterioration is considerably severe, then the etalon for stabilizing the beam quality is disabled from tracking control, and thus the laser light source gets down. Consequently, an operator is at the trouble of operating for start of the laser light source and recommencing an operation of the stepper in each occasion.